<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>直角三角形</title>
  <style>
    .box {
      width: 0;
      height: 0;
      line-height: 0;
      font-size: 0;

      border-right: 50px solid green;
      border-top: 100px solid transparent;
    }
  </style>
</head>

<body>
  <div class="box"></div>
</body>

</html>